Resistive random-access memory with protected switching layer

ABSTRACT

Resistive RAM (RRAM) devices having increased reliability and related manufacturing methods are described. Greater reliability of RRAM cells over time can be achieved by avoiding direct contact of metal electrodes with the device switching layer. The contact can be avoided by cladding the switching layer in a material such as silicon or using electrodes that may contain metal but have regions that are adjacent the switching layer and lack free metal ions except for possible trace amounts.

REFERENCE TO RELATED APPLICATIONS

This Application is a § 371 of PCT/US16/66618, filed Dec. 14, 2016, which claims priority of and incorporates by reference each of the following provisional patent applications:

-   -   U.S. Prov. Ser. No. 62/266,947 filed Dec. 14, 2015;     -   U.S. Prov. Ser. No. 62/351,632 filed Jun. 17, 2016;     -   U.S. Prov. Ser. No. 62/351,952 filed Jun. 18, 2016;     -   U.S. Prov. Ser. No. 62/351,999 filed Jun. 19, 2016;     -   U.S. Prov. Ser. No. 62/393,571 filed Sep. 12, 2016; and     -   U.S. Prov. Ser. No. 62/415,416 filed Oct. 31, 2016.         Additionally, this patent application is related to and         incorporates by reference each of the following applications:     -   U.S. Pat. No. 9,515,262 granted on Dec. 6, 2016 having U.S.         application Ser. No. 14/829,327 filed Aug. 18, 2015;     -   International Patent Application Ser. No. PCT/US2015/45731 filed         Nov. 25, 2015;     -   International Patent Application Ser. No. PCT/US2014/039990         filed May 29, 2014;     -   U.S. application Ser. No. 15/010,450 filed Jan. 29, 2016;     -   U.S. Prov. Ser. No. 61/828,667 filed May 29, 2013;     -   U.S. Prov. Ser. No. 61/859,764 filed Jul. 29, 2013;     -   U.S. Prov. Ser. No. 62/100,028 filed Jan. 5, 2015;     -   U.S. Prov. Ser. No. 62/112,159 filed Feb. 4, 2015;     -   U.S. Prov. Ser. No. 62/132,507 filed Mar. 13, 2015;     -   U.S. Prov. Ser. No. 62/137,282 filed Mar. 24, 2015;     -   U.S. Prov. Ser. No. 62/144,328 filed Apr. 7, 2015;     -   U.S. Prov. Ser. No. 62/145,450 filed Apr. 9, 2015;     -   U.S. Prov. Ser. No. 62/151,394 filed Apr. 22, 2015;     -   U.S. Prov. Ser. No. 62/171,209 filed Jun. 4, 2015; and     -   U.S. Prov. Ser. No. 62/172,110 filed Jun. 7, 2015.

FIELD

This patent specification generally relates mainly to resistive random-access memory devices. More particularly, some embodiments relate to resistive random-access memory formed with pathway structures such as implanted and/or radiated channels for enhancing mobility of charged species, without the need for a forming voltage. Still more particularly, some embodiments relate to improving reliability and other desirable properties of such random-access memory devices by keeping undesirable migration of ions into a switching layer.

BACKGROUND

In the manufacture of a known resistive random-access memory (RRAM, ReRAM or memristor) device a switching layer, typically a transition metal oxide is positioned between a top electrode and bottom electrode. The bulk switching layer is initially non-conducting. By applying a sufficiently large voltage (a “forming voltage”) across the top and bottom electrodes, haphazard conduction path(s) can be formed within the bulk switching layer. The forming voltage, which generally depends on material quality and thickness of the switching layer(s), can equal the breakdown voltage and can range from a few volts to tens of volts. Once the conduction paths are formed, they may be reset (broken, resulting in higher resistance) or set (re-formed, resulting in lower resistance) by an appropriately applied voltage (the “switching voltage”). The forming process and the haphazard nature of the path for the charged species to migrate under an applied electric field are not desirable features for large-scale memory/computing arrays for system applications.

Significant variations are often observed in current and voltage characteristics of the known RRAM. Such variations include both variations in switching voltage and variations in the resistivity of the states (high resistance state (HRS) and low resistance state (LRS). The variations are often exhibited between different RRAM devices within the same chip. The variations are highly undesirable from a system point of view, as an algorithm and possibly equipment need to be developed to interrogate each memory to determine its operating point dynamically for a single memory element. Further, changes in switching characteristics can occur for a given device over a period of time. Such changes over time of the switching characteristics are additionally troublesome since they may contribute to errors later during the life cycle of the device.

The subject matter claimed herein is not limited to embodiments that solve any specific disadvantages or that operate only in environments such as those described above. Rather, this background is only provided to illustrate one exemplary technology area where some embodiments described herein may be practiced.

SUMMARY

According to some embodiments, a resistive random access memory device (RRAM) comprises a switching layer that in operation of the RRAM changes between higher resistance state (HRS) and lower resistance state (LRS) upon application across the layer of a switching voltage that is below a breakdown voltage. A first electrode is at one side of the switching layer and a second electrode is at an opposite side of the switching layer. The electrodes are positioned to apply the switching voltage in the operation of the RRAM. Importantly, the electrodes are configured to keep migration of ions from the electrodes to the switching layer from defeating reliability or reproducibility of the RRAM.

The density of metal ions in the switching layer due to intentionally introduced ions and ions that have migrated during an expected lifetime of the RRAM is at a selected level in the range 10¹³ to mid-10²² ions/cm³. In one example, the selected level is no greater than mid-10²² ions/cm³. The electrodes can be made of material from which metal migration to the switching layer is sufficiently low to maintain the density of metal ions in the switching layer below said selected level. The switching layer initially is substantially stoichiometric except for said metal ions that are at a density below the selected level.

According to some embodiments, a respective cladding layer is positioned between each of the electrodes and the switching layer, spacing the electrodes from the switching layer, and thus inhibiting migration of ions from the electrodes to the switching layer. The electrodes can be spaced from edges toward centers of sides of the cladding layers, thus reducing undesirable electric field effects at said edges.

According to some embodiments, the switching layer is SiO₂ and the cladding layers are Si. In other embodiments, the switching layer comprises a plurality of layers of respective different materials. The switching layer can be between 1 and 100 nanometers (nm) thick and each cladding layer can be between 2 and 2000 nm thick.

If a cladding layer is between at least one of the electrodes and the switching layer, the cladding layer can be doped to create a PN junction. The materials of the switching layer and cladding layer can be selected to meet the purity standards of commercial CMOS processes.

The switching layer, and any cladding layers and electrodes, can be divided into an array of individually addressable RRAM elements.

At least one of the electrodes can comprise a metal or a metal alloy and can have a side facing and in contact with the switching layer and a region bound by said side, wherein free metal ions are absent from said region except for possible trace amounts. In this case, the RRAM need not have a separate cladding layer between the electrode and the switching layer.

According to some embodiments, a resistive random access memory device (RRAM) comprises a switching layer that switches between higher resistance state (HRS) and lower resistance state (LRS) upon application across the layer of a voltage that is below a breakdown voltage; a first cladding layer at one side of the switching layer and a second cladding layer at an opposite side of the switching layer; and a first electrode on the first cladding layer and a second electrode on the second cladding layer; wherein said electrodes are out of physical contact with the switching layer. The electrodes can comprise metal or metal alloy.

The switching layer can be substantially stoichiometric except for metal ions introduced in initially stoichiometric material in manufacturing the RRAM to cause the switching between the HRS and LRS states and any ions migrating to the switching layer from the electrodes or the cladding layers during operation of the RRAM. The density of said ions in the switching layer is below mid-10²² metal ions/cm³. The switching layer can be SiO₂ and the cladding layers Si. The switching layer can comprise a stack of layers of different materials. At least two layers in the stack can differ in thickness from each other. At least two layers in the stack can comprise oxides.

The switching layer can comprise switching ions that directly participate in the switching between HRS and LRS stages, and non-switching ions that do not directly participate in the switching but may cause damage or vacancies in the switching layer. The cladding layers can contain no more than 10¹³ impurity atoms or ions per cm³ prior to introduction of switching ions into the switching layer and excluding p and n doping ions in the cladding layers. The switching layer can be between 1 and 100 nm thick and each cladding layer between 2 and 2000 nm thick.

According to some embodiments, a resistive random access memory device (RRAM) comprises a switching layer that switches between higher resistance state (HRS) and lower resistance state (LRS) upon application across the layer of a voltage below a breakdown voltage, wherein the switching layer is substantially stoichiometric except for ions introduced in manufacturing the RRAM that are sufficient to cause the layer to switch between the HRS and LRS states and ions introduced due to operation of the RRAM. A first cladding layer is at one side of the switching layer and a second cladding layer is at an opposite side of the switching layer. A first electrode is on the first cladding layer and a second electrode on the second cladding layer. The total density of ions in the switching layer causing switching between the HRS and LRS states, including ions introduced in manufacturing and ions migrated from the electrodes or the cladding layers in operation of the RRAM, does not exceed 10²² ions/cm³. The switching layer can comprise two or more layers of different materials, which can have different densities of metal ions. At least one of the electrodes can comprise a metal or a metal alloy and is spaced from the switching layer by a distance inhibiting migration of metal ions to the metal or metal alloy to the switching layer. The cladding layers' impurities can be at a level conforming to commercial CMOS impurity standards, or at a level lower that commercial CMOS impurity standard. The switching layer thickness can be in the range of 1 to 100 nm, and each of the cladding layers can be between 2 and 2000 nm thick.

The switching layer can comprise one or more oxides selected from the group of silicon dioxide, aluminum oxide, titanium oxide, tantalum oxide, hafnium oxide, vanadium oxide, tungsten oxide, molybdenum oxide, niobium oxide, chromium oxide, nickel oxide, transition metal oxides, silicon nitride, niobium nitride, tungsten nitride, gallium nitride, aluminum nitride, all metal nitrides, silicon carbide, hafnium carbide, magnesium carbide transition metal carbide, and metal carbides.

At least one of the cladding layers can comprise one or more material selected from the group consisting of Si, Ge, SiGe, GaAs, GaN, AlN, InP, InN, AlGaN, and the III-V material family.

The electrodes can include non-trace amounts of one or more of Al, aluminum silicide, platinum silicide, nickel silicide, titanium silicide, magnesium silicide, tungsten silicide, molybdenum silicide, platinum, titanium, tungsten, tantalum, copper, and other metals.

BRIEF DESCRIPTION OF THE DRAWINGS

To further clarify the above and other advantages and features of the subject matter of this patent specification, specific examples of embodiments thereof are illustrated in the appended drawings. It should be appreciated that these drawings depict only illustrative embodiments and are therefore not to be considered limiting of the scope of this patent specification or the appended claims. The drawings illustrating devices are not to scale and show in straight lines surfaces and interfaces that in actual devices would not be as linear or planar. The subject matter hereof will be described and explained with additional specificity and detail through the use of the accompanying drawings in which:

FIGS. 1A-1D are diagrams showing structures for use in manufacturing nonvolatile memory elements, the structures including an insulator SiO₂ clad above and below with Silicon, according to some embodiments;

FIG. 2 is a schematic cross section of a nonvolatile memory element after ion implantation and/or irradiation showing possible regions with ions and/or defects, according to some embodiments;

FIG. 3 is a schematic cross section of a nonvolatile memory element and illustrates implanted ions for switching, according to some embodiments;

FIG. 4 is a schematic cross section of a nonvolatile memory element with switching ions and ions used to generate defects, according to some embodiments;

FIG. 5 is a schematic cross section of a nonvolatile memory element having a switching layer made of silicon oxide, hafnium oxide, silicon nitride, silicon carbide, titanium oxide, or tantalum oxide, according some embodiments;

FIG. 6 is a schematic cross section of a nonvolatile memory element and illustrates switching layers having complex compositions, according to some embodiments;

FIG. 7 is a schematic cross section of a nonvolatile memory element and illustrates further aspects of switching layer compositions, according to some embodiments;

FIG. 8 is a schematic cross section of a nonvolatile memory element and illustrates further aspects of switching layer compositions, according to some embodiments;

FIG. 9 is a schematic cross section of a nonvolatile memory element having a silicon dioxide switching layer doped with Hf and O, according to some embodiments;

FIG. 10 is a schematic cross section of a CBRRAM or CBRAM structure where no metal electrodes are in direct contact with the host material, according to some embodiments; and

FIGS. 11A-11C and 12A-12B are cross section drawings illustrating aspects of manufacturing memory cells, according to some embodiments.

DETAILED DESCRIPTION

A detailed description of examples of preferred embodiments is provided below. While several embodiments are described, it should be understood that the new subject matter described in this patent specification is not limited to any one embodiment or combination of embodiments described herein, but instead encompasses numerous alternatives, modifications, and equivalents. In addition, while numerous specific details are set forth in the following description in order to provide a thorough understanding, some embodiments can be practiced without some or all of these details. Moreover, for the purpose of clarity, certain technical material that is known in the related art has not been described in detail in order to avoid unnecessarily obscuring the new subject matter described herein. It should be clear that individual features of one or several of the specific embodiments described herein can be used in combination with features or other described embodiments. Further, like reference numbers and designations in the various drawings indicate like elements.

In known resistive random access memory (RRAM or ReRAM), also known as “memristors,” the RRAM are fabricated by depositing a switching layer that is sub-stoichiometric and/or non-stoichiometric and often with defects, and the switching layer is in direct contact with metal electrodes. Ions can be implanted into such switching layers. See, e.g., The effect of Ti and O ion implantation on the resistive switching in Pt/TiO _(2-x) /Pt devices; Gao et al, Appl. Phys. A (2015) 120:1599-1603 (incorporated herein by reference), which discusses implanting Ti and O through the top Au/Pt electrode into a non-stoichiometric TiO_(2-x) switching layer deposited by atomic layer deposition (ALD). Implanting Ti and O ions resulted in reduction or elimination of the forming voltage. However, starting with a non- or sub-stoichiometric switching layer having defects and introducing additional ions that may further change its stoichiometricity and an increase in defects due to ion implantation can result in non-reproducible resistive switching characteristics. Starting with a defective film and introducing more defects and uncertainties in the stoichiometricity of the switching layer can result in large variations in the RRAM characteristics and poor reproducibility. In addition, in such known RRAM the switching layer is in direct contact with metal such as Pt/Ti and thermal annealing is used to cause the Ti to diffuse into the switching layer. This results in further non-stoichiometricity and hard-to-control diffusion of Ti ions, which further adds uncertainties and non-reproducibility. The density of defects and ions and vacancies are unknown. This is similar to not knowing and/or not controlling the doping of a pn junctions with acceptors and donor atoms that can result in a non-reproducible transistor for example. Improvement of Resistive Switching Properties in ZrO ₂-Based ReRAM With Implanted Ti Ions, Liu et al, IEEE ELECTRON DEVICE LETTERS, VOL. 30, NO. 12, DECEMBER 2009 (incorporated herein by reference) discusses a switching layer of ZrO₂ which is electron beam evaporated and subsequently thermally annealed resulting in a polycrystalline layer with defects and regions of non-stoichiometricity. In addition the ZrO_(x) is deposited on metal which results in poor interface quality and uncontrolled metal ion diffusion into the switching layer resulting in defects and non-stoichiometric regions and unknown and/or uncontrollable doping of metal ions in the switching layer. Ti ions are then implanted into the ZrO_(x) switching layer adding Ti ions, and defects. With so many uncontrollable and unknown variables such as density of metal ions, oxygen ions, vacancies, defects it is difficult to attain reproducibility, repeatability, device to device consistency of the switching parameters and reliability to CMOS manufacturing and performance standards. In Application of ion-implantation for improved non-volatile resistive random access memory (ReRAM), Elliman et al, Nuclear Instruments and Methods in Physics Research B 307 (2013) 98-101 (incorporated herein by reference), oxygen is implanted into a stoichiometric Ta₂O₅ film to introduce extra O ions and vacancies for switching and the Ta₂O₅ transforms into a sub-stoichiometric layer TaO_(x). Ta electrodes are used to contact the switching layer directly. The metal ions in the metal electrode can diffuse readily into the switching layer during growth of the switching layer and during device operation where voltage and electric current exist. Uncontrolled metal ions, defects and vacancy concentration can affect reproducibility, reliability and variability of the RRAM devices switching characteristics. Electric current during device operations can drive metal ions from the electrode that is in direct contact with the switching layer into the switching layer and can cause unknown and uncontrollable variation in density of the metal ions within the switching layer. Irreproducibility, poor reliability and variability can result both within a single device and from device to device.

It has been proposed in known technology that a sub-stoichiometric switching layer is needed for resistive switching and in particular for forming free resistive switching. See e.g., Forming-free resistive switching characteristics and improved reliability in sub-stoichiometric NbN _(x) films, Kim et al, Phys. Status Solidi RRL 9, No. 4 (2015) (incorporated herein by reference). Ti and Pt/Ti electrodes are in direct contact with the NbN_(x) switching layer, which can cause diffusion of metal ions into the switching layer causing poor reliability and non-reproducibility of the RRAM switching characteristics.

According to some embodiments of this disclosure, the starting switching layer is as stoichiometric and with as few defects as possible or at least practicable. It is desirable to have the switching layer be stoichiometric and crystalline or almost crystalline. In addition, the switching layer interface should not be to a metallic surface that can cause uncertainties of metal ion diffusing into the switching layer. Such diffusion can be difficult to control and reproduce on a manufacturing scale. Direct contact with metal surfaces by the switching layer interface can cause metal ion diffusion due to not only high temperature processing/fabrication but also during actual operation of the RRAM since local heating due to the product of current and voltage (joule heating) power dissipation can be as high if not higher than some processing temperatures. See, e.g., The switching location of a bipolar memristor: chemical, thermal and structural mapping, Strachan et al, Nanotechnology 22 (2011) 254015 (6pp) (incorporated herein by reference). Such diffusion of metal ions into the switching layer during actual device memory operations can cause additional defects. The migration of additional ions can eventually lead to device failure after many cycles of HRS-LRS-HRS (High Resistive State-Low Resistive State) memory operation of the RRAM device. The HRS/LRS ratio can diminish and eventually the device can be electrically shorted indicating excess conductive paths and/or excess conductivity which can be caused by excess metal ions, and/or excess defects that can provide current leakage paths. Direct contact of the switching layer with metal electrodes that can diffuse metal ions into the switching layer is not desirable. In contrast, diffusion barriers, interfaces where diffusion is slow, low diffusivity, difficult and/or minimal diffusion or drift of the metal ions into the switching layer, lead to higher reproducibility and reliability of the RRAM switching characteristics. Diffusion and drift are two methods by which metal ions can migrate into the switching layer, diffusion is temperature dependent and drift is electric field dependent. See, e.g., Metal-Dielectric Diffusion Processes: Fundamentals, chapter 2, He et al., Metal-Dielectric Interfaces in Gigascale Electronics, 11 Springer Series in Materials Science 157, DOI: 10.1007/978-1-4614-1812-2_2, Springer Science+Business Media, LLC 2012 (incorporated herein by reference).

FIGS. 1A-1D are diagrams showing structures for use in manufacturing nonvolatile memory elements, the structures including an insulator SiO₂ clad above and below with Silicon, according to some embodiments. In FIG. 1A The SOI (Silicon On Insulator) or BOX (Buried OXide) structure includes an SiO₂ layer 120 clad both above and below with silicon layers 122 and 124. According to some embodiments, an oxygen ion implanted buried layer in Silicon can be annealed to form buried silicon dioxide layer 120 as is shown in FIG. 1B. According to some other embodiments, the upper Si layer 122 can be formed by epitaxial lateral overgrowth on the silicon dioxide layer 120 as shown in FIG. 1C. In this case both the top and bottom Si layers 122 and 124 are device quality and in some cases CMOS device quality. Other buried insulating layers such as silicon nitride, silicon carbide, are also possible instead of, or in addition to SiO₂ layer 120. In the case of epitaxial overgrowth, other insulating layers such as tungsten oxide, tantalum oxide, titanium oxide, to name a few, can be grown stoichiometric and/or almost stoichiometric by methods such as atomic layer deposition for example. Metal ions for switching can also be introduced controllably during atomic layer deposition such that an accurate doping of the metal ions and/or oxygen ions can be grown into the switching layer accurately in both doping level and spatial distribution. Atomic layer deposition is one possible method of accurate growth, some other methods may include molecular beam epitaxy, plasma enhanced chemical vapor deposition, and laser ablation deposition. The buried insulating and/or switching layer can range in thickness from 1 nm to 100 nm (nanometers), and the silicon layer cladding the insulating layer can range in thicknesses from 2 nm to 1000 nm or more. The silicon layers 122 and 124 can be doped p and/or n type, and/or intrinsic or not intentionally doped. For example, the top Si layer 122 can be doped p type and the bottom Si layer 124 can be doped n type. In some cases the top Si 122 can be doped n type and the bottom Si 124 doped p type. In some other cases, both top and bottom Si are doped p or n of same or different doping level. In some further cases the top can be doped p or n and the bottom is intrinsic. In some further cases the bottom can be doped p or n and the top Si can be intrinsic. In some further cases both top and bottom Si layers are intrinsic. In some cases, both top and bottom can be doped n or doped p type. The p and n doping levels can range from 5×10¹⁴/cm³ to 5×10²¹/cm³ or more. In some cases, the Si layers can have both p and n doping to form a pn junction for example.

Other insulating layers can be used. In the case of SiO₂, it is desirable for the layer to be as stoichiometric as possible or practicable and as defect free as possible or practicable. In some cases the SiO₂ 120 is of CMOS quality and the silicon layers below and above the insulator are as defect free as reasonably possible and of CMOS quality. The SiO₂ layer 120 can be approximately 1-100 nm thick and the Si top of the insulator can be approximately 2-2000 nm thick.

Ion implantation and/or radiation is then performed on the SiO₂ layer 120 and necessarily passing through the top Si layer 122 and in some cases to the bottom Si layer 124, either locally where an implant mask can be used or globally. Implanted ions can be a combination, single and/or multiple different species of ions from the transition metal family, non-transition metal families and/or O, Ne, Ar, H, N, B, C, P. For example, Ti and/or O can be implanted partially into or through the SiO₂ layer where defects and ions controlled in both density and spatial distribution, e.g., Ti, Ta, Nb, O and/or N ions, can contribute to RRAM switching characteristic with high reproducibility and reliability and low variability in the RRAM switching parameters. Ion implantation and/or radiation together with or without rapid thermal annealing can be used to more precisely control the defect density and spatial distribution and thereby the switching parameters of the RRAM such as set and reset voltages, and preferably with low and/or no forming voltages.

Ion implantation of metal, oxygen, argon, and/or nitrogen can be used in conjunction with doping of the switching layers with metal ions and/or oxygen ions during growth such as by atomic layer deposition.

The techniques described herein when compared to known techniques with or without ion implantation differ by initially using a layer that a high quality defect free (or nearly defect free, such as is suitable for CMOS applications) ed that initially does not exhibit switching characteristics of multiple high resistance state (HRS) to low resistance state (LRS) cycles, and by using ion implantation and/or radiation to implant the switching ion(s) and controlled damages that can result in a reproducible, reliable switching characteristics of a RRAM device with low variability in its switching parameters. In addition, metal ions, oxygen and/or nitrogen can be incorporated during growth of the switching layer(s) when switching layers are fabricated by a growth method such as atomic layer deposition, and Si is epitaxially over grown on the switching layers. The switching layers can be isolated by a trench etch 140 and/or ion implantation as shown in FIG. 1D for example. Furthermore, metal electrodes are not in direct contact with the switching layer(s). In this example, Silicon 122 and 124 is in contact with SiO₂ layer 120 and the metal electrodes (see, e.g., FIG. 2) can be in contact with the Silicon 122 and 124.

One embodiment can be that the metal electrodes are not present during growth of SiO₂ layer on Si, ion implantation, or high temperature annealing or processing that could cause uncontrollable metal ion diffusion into the switching layer. The switching layer is the SiO₂ layer 120, metal ions, O, N, H, B, C ions, inert ions such as Ne, Ar, Kr, Xe, and other ions in the periodic table that are implanted into and through the SiO₂ layer. For example, Ta ions and O ions can be implanted into and/or through the SiO₂ layer 120 to initiate resistive switching characteristics of a RRAM device with or without thermal annealing. Another example is Nb and N ions implanted into and through the SiO₂ layer 120 together with other ions such as Ar for example with or without post-implant thermal annealing.

According to some embodiments of this disclosure, the resistive switching (the terms switching and resistive switching are used herein interchangeably) layer is the initial SiO₂ layer 120 (or equivalent layer in nitride, carbide, hydride) bounded by Si 122 on the top and Si 124 on the bottom. The SiO₂ layer 120 also includes implanted ions such as transition metal ions, vacancies, oxygen and/or nitrogen to create regions in the SiO₂ and/or Si that are non- and/or stoichiometric. The SiO₂ layer 120 has controlled defects, ions, vacancies, densities and spatial distribution that can cause resistive switching. Si layer 122 and/or 124 can also be implanted as needed for tuning the stoichiometricity of the SiO₂ after and/or during the metal and O and/or N implant or for other reasons such as doping or creating PN junctions in one or both layers 122 and 124.

In addition, the interfaces of the switching layer are with a non-metallic surface, in this example, a Si—SiO₂—Si interface such that metal ions from electrodes are kept a distance from the switching layer and therefore difficult to diffuse/migrate to the switching layer. An example of metal electrodes is shown in FIG. 2, where the electrodes are placed on the Si surface.

According to another embodiment of this disclosure, a high dielectric constant material such as HfO₂ is used as the initial switching layer and is bounded on both top and bottom interfaces by polysilicon, and/or amorphous silicon and/or crystalline silicon for example as part of the top and bottom electrodes. In some cases, the hafnium oxide can be “buried” using epitaxial lateral overgrowth as in FIGs. 1C where the silicon dioxide in layer 120 can be replaced with hafnium oxide. After epitaxial lateral overgrowth, a chemical mechanical polishing step, commonly used in CMOS fabrication, can be used to polish the Si surface to the surface finish of CMOS fabrication processes specifications.

Ion implantation can be used to implant transition metal ions, O and/or N to cause the HfO₂ layer to be sub- or non-stoichiometric and result in oxidation-reduction type resistive switching due to formation of filaments, percolation paths, hopping, and other methods of charge transport. Metal electrodes are not in direct contact with the switching layer. Other oxides, nitrides, carbides and non-metal electrodes interfaces can be used. Metal electrode can be added at interfaces that are not the switching layer interface, for example, Si—Al can be added at the polysilicon interface.

Other switching layers and dopants are also possible. See, e.g.: Doping Technology for RRAM—Opportunities and Challenges, Magyari-Köpe et al., 978-1-4673-9478-9/16/$31.00 ©2016 IEEE; Dopant concentration dependent resistive switching characteristics in Cu/SiNx/Si structure, Kim et al., Journal of Alloys and Compounds 686 (2016) 479-483 (referred to hereinafter as “Kim et al.”); and Nature of Cu Interstitials in AlO and the Implications for Filament Formation in Conductive Bridge Random Access Memory Devices, Dawson et al., J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b02728 • Publication Date (Web): 13 Jun. 2016 Downloaded from http://pubs.acs.org on Jun. 19, 2016 (referred to hereinafter as “Dawson et al.”), all three of which are incorporated herein by reference. Examples of Metal-oxide RRAMs are given in Metal-Oxide RRAM, Wong et al., Vol. 100, No. 6, June 2012|Proceedings of the IEEE (incorporated herein by reference). In known techniques, a metal electrode and/or metal containing alloy electrode such as TiN is in direct contact with the switching layer. This is in contrast with embodiments of the present disclosure wherein the layers cladding the switching layer are not initially formed of either a metal or a metal-containing alloy. However, metal ions may be implanted into the cladding layers in addition to the switching layer after forming the cladding and switching layers. In addition, metal ions such as transition metals and post-transition metals in the switching layer deposited during growth for example may diffuse into the cladding layers that did not initially contain the metal ions intentionally. In addition, the cladding layers can be doped with acceptors and/or donors such as B, P and As to create p and n type semiconductors and/or pn junctions on the top and/or bottom Si cladding layer(s).

FIG. 2 is a schematic cross section of a nonvolatile memory element after ion implantation and/or irradiation showing possible regions with ions and/or defects, according to some embodiments. Rapid thermal annealing in Ar for example can be used to remove some of the defects to more precisely control the RRAM switching parameters. Other annealing techniques such as laser annealing can be used for more localized anneal for example. The Si layer 222 on top of SiO₂ or any other electrically insulating layer 220, labeled Si-Top (T) can also include a pn junction to provide a diode characteristic in the IV (current voltage) in order to reduce or minimize sneak current. Using standard photolithographic methods, the Si-Top (T) 222 can be patterned and a top electrode TE 210, ohmic and/or non-ohmic, can be deposited on the Si-Top (T) 222 preferably after ion implantation and in some cases before ion implantation. The top and/or bottom electrodes 210 and 230 can be metal, silicide such as PtSi, AlSi, TaSi, WSi, for example, and/or conductive nitride such as TiN and/or conductive oxide such as indium tin oxide. The substrate, which in this example is the bottom silicon labeled Si-Bottom (B) 224, can be chemically mechanically polished (CMP), patterned and metalized into a crossbar structure. Intermediate steps such as bonding the RRAM wafer to a sacrificial wafer for CMP processing and transferring the RRAM crossbar to an ASIC CMOS (application specific integrated circuit) wafer, and many other steps are omitted for simplicity as they can be similar to known IC processing. The dashed lines 250 represent defects caused by ion implantation and/or irradiation. The implant and/or irradiation for the structure shown in FIG. 2 can be selective or local such that only a specific region is implanted/irradiated as compared to blanket or global implant/irradiation. The localized implant/irradiation confines the switching away from the edge of the Si-Top (T) 222 where high electric field may exist and can cause reliability issues.

In this example, Si cladding layers doped either as n and/or p and/or pn are adjacent to the switching layer SiO₂ that was implanted with transition metal ions and/or O or N for example and with controlled defect density and spatial distribution with or without radiation and with or without thermal anneal. The interface between Si and the insulating SiO₂ layer is mostly lattice-matched and of CMOS quality and without metal in direct contact with the SiO₂ layer. Metal and/or silicide and/or TiN contacts are either Schottky or ohmic and contact the Si layers 222 and 224 as shown in FIG. 2 as TE 210 and BE 230. The electrodes can be Pt, W, Ta, Ti, Ni, Cu, Al, Cr, TiN, PtSi, AlSi, NiSi for example.

The metal ions taking part in the resistive switching mechanism are ion implanted and/or exist as part of the stoichiometric switching layer (for example HfO₂ switching layer, where the Hf may take part in the switching process in addition to implanted ions (that may also be Hf for example) and/or ions doped during switching layer growth process. Such metal ions have not diffused/drifted from metal and/or metal alloy electrodes in direct contact with the switching layer(s). This technique has been found to lead to greater reproducibility, repeatability and reliability of the RRAM devices.

FIG. 3 is a schematic cross section of a nonvolatile memory element and illustrates implanted ions for switching, according to some embodiments. The structure is as shown in FIG. 2, and the white and black circles 350 represent implanted ions, possibly of different types, for switching such as transition metals and oxygen and/or nitrogen for example. Damage from implanting of ions for switching such as transition metal ions such as Ti, Ta, W, Cr, Cu, Ni, Zn, Zr, Mn, Fe, V, Hf, Nb, Mo, non-transition metals such as Be, Mg, Ga, In, Bi, Se, Te, and/or non-metals such as O, N, H, C, F, Cl, Ne, Ar, Xe can be mostly annealed out using rapid thermal annealing. See e.g., Optimal control of rapid thermal annealing in a semiconductor process, Gunawan et al., Journal of Process Control 14 (2004) 423-430 (incorporated herein by reference). The residual damage if any, and/or the stress in the SiO₂ layer due to the addition of the ions with doping ranging from 10¹³/cm³ to as high as mid 10²²/cm³ can be sufficient to result in forming free switching. Multiple energy ion implantation can be used if desired to generate a distribution of ions and/or multiple types of ions for better or optimal switching. In one embodiment, the distribution is mostly uniform for example Hf and O ions, Ti and O ions, Ta and O ions or Nb and N ions. In another embodiment, the O and/or N ions and the transition metal ions for example can be spatially separated with some overlap. This controlled introduction of switching ions and the purity of the process and/or using CMOS grade oxide layer as the switching layer matrix can result in high reproducibility and reliability of the RRAM memory system.

In some cases, the implanted ions are confined predominately in the switching layer 220 and in some cases predominately outside the switching layer 220, and in some cases, can penetrate and/or straggle into the Si cladding layer(s) 222 and 224. In some cases implanted ions can be in both the switching layers 220 and one or both of the silicon cladding layers 222 and 224. In some cases, the damage due to ion implant and/or radiation can exist in the switching layer 220 and/or one or both silicon cladding layer(s) 222 and 224. The damage from the ion implantation and/or radiation can be partially annealed out and/or fully annealed out to improve switching performance.

Impurities can be introduced into the SiO₂ layer or other oxide layers or nitride layers during the fabrication of the buried oxide layer by implantation and/or other deposition methods such as atomic layer deposition where the switching ions of metal and/or oxygen and/or nitrogen are introduced as dopants during growth of the switching layer for example. In certain cases, ion implantation and or radiation may or may not be further necessary for reliable and reproducible switching where the switching layer can be stoichiometric and or non-stoichiometric.

In addition, the switching layer can be a gate oxide such as hafnium oxide in CMOS and the bottom electrode can be highly doped Si and the top electrode can be highly doped polysilicon and or amorphous silicon and/or crystalline silicon, similar to a CMOS transistor but without the source and drain. Ions can be implanted through the polysilicon and/or silicon, where it can be thinned, to provide switching behavior. The gate oxide can be stoichiometric oxide such as silicon dioxide, hafnium oxide, where the oxide in the case of Si can be grown thermally and/or by atomic layer deposition for example. Starting with stoichiometric and/or crystalline switching layer similar to CMOS gate oxides and/or buried oxide layers and then introducing precisely and with high purity the switching ions can give good resistive memory characteristics. Prior to the introduction of the ions, the switching layer will not exhibit resistive switching characteristics reliably and reproducibly and will need to first reach a breakdown voltage before exhibiting desirable switching characteristics and in many cases may not switch more than once for example from high resistive state to low resistive state and stay at low resistive state.

FIG. 4 is a schematic cross section of a nonvolatile memory element with switching ions and ions used to generate defects, according to some embodiments. The switching ions and ions used to generate defects can be implanted concurrently and/or sequentially where a thermal annealing such as rapid thermal annealing can be used between ion implantation processes. For example, thermal annealing can be used after the switching ion implant and the defect generating ions can be implanted after the thermal anneal and in some cases a thermal anneal may not be used at all. The switching layer 220, as shown in the FIG. 4, is between silicon layers 222 and 224 that can be doped n and/or p type and/or undoped. The silicon layer thickness can range from a few nanometers to hundreds of nanometers or more and in some cases from 1 nm to 2000 nm or more. Metal and/or conductive metal nitride TE 210 and BE 230 can be standard metal used in CMOS circuits such as aluminum and/or TiN for example and/or silicide and/or amorphous silicon that can be doped or undoped.

FIG. 5 is a schematic cross section of a nonvolatile memory element having a switching layer 520 made of silicon oxide, hafnium oxide, silicon nitride, silicon carbide, titanium oxide, or tantalum oxide, according some embodiments. Layer 520 can be made of other material as well and is stoichiometric or almost stoichiometric. The switching layer 520 is between silicon layers 522 and 524 that can be crystalline, polycrystalline and/or amorphous Si and that can be doped n and/or p and/or undoped. The switching layer structure 520 can be deposited using techniques such as atomic layer deposition, plasma enhanced chemical vapor deposition, laser ablation deposition and/or molecular beam epitaxy, with thickness ranging from 0.1 to 100 nm or more. The switching layer 520 can be composed of multiple layers of dielectric material and each layer can have the same and/or different thicknesses. For example silicon oxide and hafnium oxide layers can be used for the switching layer 520. The switching ions 350 concentration can be controlled precisely by ion implantation and/or doping during switching layer growth which can be important for reproducibility and reliability. Doping during switching layer growth can result in a non-stoichiometric layer where the switching ions dope and disrupt the stoichiometricity of the switching layer. See, Wafer bonding for silicon-on-insulator technologies, Lasky, Applied Physics Letters 48, 78 (1986) (incorporated herein by reference). The Si can be wafer bonded to the switching layer(s) where a thin layer of SiO₂ can be used at the interface between the Si and the switching layer. The thin layer of SiO₂ can be doped with switching ions and/or non-switching ions. For example, the top layer is doped with Ti in SiO₂ and the bottom layer is not doped. In other examples both the top and bottom layers can be doped with switching and/or non-switching ions and/or extra oxygen ions for oxidation and reduction based switching mechanisms. Doping of switching ions can be uniform in the switching layer(s) and in some cases the ions are not uniformly distributed, in some cases the ions partially distributed within a switching layer(s), and in some cases the ions can have multiple distributions that can be uniform and/or not uniform and/or a combination thereof.

In addition, ion implantation as shown in FIG. 5 can be implemented to generate defects 250 in the crystalline and/or non-crystalline switching layer(s) to facilitate the formation of conductive channels due to species migration and mobility under electric and/or thermal gradient and/or other gradient such as stress such that a breakdown voltage can be omitted in the operation of the nonvolatile memory. The implant ions can be switching ions and/or non-switching ions such as Ar, N, H, C, Ne, Au, to name a few.

Switching ions are introduced into the switching layer by ion implantation (e.g. ions 350) and/or by doping during switching layer growth and not by diffusion of switching ions from electrode in direct contact with the switching layer as in known prior art.

FIG. 6 is a schematic cross section of a nonvolatile memory element and illustrates switching layers 620 having complex compositions, according to some embodiments. In switching layers 620, the switching ions are added precisely during growth such as with atomic layer deposition for example. The thickness of each layer ranges from 0.1 nm to 30 nm or more, where each of the layers can have a single thickness and/or multiple thicknesses. Single and/or multiple types of ions as either dopant and/or composition can be included with each layer. For example the top layer can be Hf oxide and Ti Oxide and silicon dioxide, another layer can be vanadium oxide and titanium oxide, other layers can be silicon dioxide and/or titanium oxide, and the bottom layer can be silicon dioxide doped with hafnium ions. There can be single and/or multiple layers 620. These layers are sandwiched by silicon 522 and 524 that can be crystalline and/or non-crystalline and formed using wafer bonding for example, or similar to the formation of SOI process or epitaxial lateral overgrowth as in FIG. 1C where Si and the oxide layer can form a bond through silicon dioxide at the interface with the oxide layer which in some cases can be a silicon dioxide layer as a part of the switching layer(s).

Ion implantation can be used to generate defects 250 with switching and/or non-switching ions, and the implant depth can be into the switching layers, partially into the switching layers and/or through the switching layers and/or partially into the bottom Si layer. Ion implantation can be conducted prior to wafer bonding and/or after wafer bonding and implanted through thinned top silicon material. Metal and/or silicide can then be used on the silicon to form electrodes to reduce resistivity of the electrically conduction paths to each nonvolatile memory element.

FIG. 7 is a schematic cross section of a nonvolatile memory element and illustrates further aspects of switching layer compositions, according to some embodiments. As in FIG. 6, switching layer(s) 720 can have different composition and/or doping of switching and/or non-switching ions. The layers 720 can be grown by atomic layer deposition for example with precise control of the introduction of species density and spatial distribution. The layers 720 are sandwiched between silicon material 522 and 524 that can be doped p and/or n and/or undoped intrinsic and can be crystalline and/or non-crystalline. Electrodes of material such as Al and/or silicide for example, similar to electrode material in CMOS process, are used to reduce resistivity of the electrodes leading to each nonvolatile memory element in this case. A resistive random access memory element also is called a memristor. Thicknesses of silicon and switching layers are as described supra. In the example of layer 720, hafnium ions and oxide of hafnium are the switching species together with oxygen. As shown, Hf ions are doped into the silicon dioxide layer(s) along with HfOx and Ox ions into various layer(s). Only a few example layers are shown. Variations of Hf, HfOx, Ox, silicon dioxide can be used and HfOx can be stoichiometric and/or non-stoichiometric and oxygen can be an ion, and/or ionized, and/or neutral. Ion implantation can be used to create defects before or after wafer bonding with switching and/or non-switching ions, and/or oxygen ions to create defects and to dope the switching layers with switching and/or non-switching ions (ions that contribute to switching such as transition metals and non-switching ions are for creation of damage/vacancies only such as Ar, He, Xe to name a few).

Other deposition methods can be used such as plasma enhanced chemical vapor deposition, chemical vapor deposition, laser ablation deposition, to name a few.

FIG. 8 is a schematic cross section of a nonvolatile memory element and illustrates further aspects of switching layer compositions, according to some embodiments. The nonvolatile memory element has switching layer(s) 820 that can have different composition and/or doping of switching and/or non-switching ions. Switching ions are ions that participate in the resistive memory process, such as metal ions and oxygen, nitrogen, hydrogen ions, and in addition can also generate damage and vacancies. Non-switching ions are ions that don't participate directly in the memory process and are used for creating damage and vacancies, such as Ar, He, Xe, and other ions that do not participate in the oxidation-reduction processes of a resistive memory and/or radiation of high energy photons for example. The layers 820 can be grown by atomic layer deposition for example with precise control of the introduction of species. The layers 820 are sandwiched between silicon material 522 and 524 that can be doped p and/or n and/or undoped and can be crystalline and/or non-crystalline. Electrode materials such as Al and/or silicide for example, similar to electrode material in CMOS process, are used to reduce resistivity of the electrodes 210 and 230 leading to each nonvolatile memory element in this case. Thicknesses of silicon and switching layers can be as described, supra. In this example, hafnium ions and oxide of hafnium are the switching species together with oxygen. As shown, Hf ions are doped into the silicon dioxide layer(s) along with HfOx and Ox ions into various layer(s). Only a few example layers are shown; variations of Hf, HfOx, Ox, silicon dioxide can be used and where HfOx can be stoichiometric and/or non-stoichiometric and oxygen can be an ion, and/or ionized, and/or neutral. Ion implantation can be used to create defects before or after wafer bonding with switching, and/or non-switching ions, and/or oxygen ions to create defects.

Other deposition methods can be used such as plasma enhanced chemical vapor deposition, chemical vapor deposition, laser ablation deposition, to name a few.

FIG. 9 is a schematic cross section of a nonvolatile memory element having a silicon dioxide switching layers 920 doped with Hf and O, according to some embodiments. In this example the switching layers 920 include a layer 940 that is doped precisely with Hf and O in a silicon dioxide layer using atomic layer deposition for example. The layers 920 are sandwiched between silicon material 522 and 524 that can be doped n and/or p and/or undoped and have metal and/or silicide electrodes in contact with the silicon. Ion implantation can be used to generate defects to facilitate the formation of conductive channels. A tunnel SiO₂ layer 942 can be included or not included. When the conductive filaments approach and come to contact the silicon material, a rectifying junction can be formed which is beneficial to reduce the sneak current effect in a crossbar RRAM architecture. In addition, with the tunnel silicon dioxide layer 942, a similar rectifying junction can be formed with the conductive filament(s).

Ion implantation can be used to introduce dopants such as metal ions, O, N that can contribute to the switching phenomenon and can create damage and vacancies and also ions such as Ar, He, Xe, that are used only to create damage and vacancies.

According to some embodiments of this disclosure, switching ions are introduced with precise doping density and spatial distribution, and the ions introduced this way, by growth and/or by ion implantation, participate in the switching phenomenon.

In two known structures Cu is deposited on a host material that may not participate directly in the switching process and is a conduit for the formation of metal ion bridges in a Conductive Bridge Random Access Memory or a Conductive Bridge Resistive Random Access Memory (CBRAM or CBRRAM). Dawson et al. and Ref. Kim et al. discuss dopant concentration dependent resistive switching characteristics in a Cu/SiNx/Si structure, where in both cases, metal such as Cu is in direct contact with the SiN or AlO switching host or layer. The switching layer or host may or may not participate in the switching process. The switching layer may contribute an O ion or vacancy and in some cases the switching host or layer may not participate and is simply a matrix to hold the switching species. Note that as used herein, the term switching layer includes, but is not limited to, host structures or host layers having bulk materials that do not directly participate in switching but rather host filaments, other structures or other materials that do participate in switching.

According to some embodiments of the present disclosure, metal electrode(s) are not in direct contact with either the switching layers and/or the host material (such as the SiN, AlO, SiO₂, to name a few). This is in contrast with known techniques where at least one metal or metal alloy such as TiN is in contact with the switching layer and/or host material.

FIG. 10 is a schematic cross section of a CBRRAM or CBRAM structure where no metal electrodes are in direct contact with the host material, according to some embodiments. The host material 1020 can be SiN, AlO and/or SiO₂ and/or combination thereof and can also include other materials such as diamond, carbide, SiN. In the case of oxidation reduction switching layers of TiO, HfO, WO, NbO, VO, TaO, the metal electrodes are not in direct contact with the switching layers or host. If metal electrodes were to be in direct contact with the switching layers and/or host layers, an uncontrollable density and number of ions would be available to participate in the switching process as the metal electrode can provide an “infinite” source of switching ions/species. This could eventually lead to failure of the nonvolatile memory device and/or result in non-reproducible switching characteristics.

According to some embodiments of the present disclosure, the metal electrode(s) do not contribute to the switching process since they are not in direct contact with the switching and/or host layer(s) which can be silicon dioxide, aluminum oxide, titanium oxide, tantalum oxide, hafnium oxide, vanadium oxide, tungsten oxide, molybdenum oxide, niobium oxide, chromium oxide, nickel oxide, transition metal oxides, silicon nitride, niobium nitride, tungsten nitride, gallium nitride, aluminum nitride, all metal nitrides, silicon carbide, hafnium carbide, magnesium carbide transition metal carbide, metal carbides. The switching layer(s) and/or host layer(s) such as 920 are sandwiched between layers 522 and 524 of Si and or other semiconducting material such as Ge, SiGe, GaAs, GaN, AlN, InP, InN, AlGaN, III-V material family to name a few, to keep away from the switching or host layer(s) metal ions from electrodes, which can be Al, Silicides such as aluminum silicide, platinum silicide, nickel silicide, titanium silicide, magnesium silicide, tungsten silicide, molybdenum silicide, platinum, titanium, tungsten, tantalum, copper, and other metals, 210 and 230 that might contribute to the switching process if one or both of those electrodes were to be in direct contact to the switching and/or host layer(s) 920. According to some embodiments, an Al electrode is deposited on the surface of the silicon opposite to the surface that is in contact to the switching and/or host layer(s). Other metal electrodes can also be used in this case, for example, Pt, Cu, Ti, Ag, Ni, to name a few.

As shown in FIG. 10, switching ions can be incorporated by doping methods such as during AlO and/or SiN growth. Cu and/or Ag can be incorporated in the growth process for example during ALD, PECVD, CVD to name a few. Such ions can also be incorporated by ion implantation of Cu and Ag for example. In addition, ion implantation of ions such as Ar, N, H, O, Ne, Au, Pt, to name a few, can be used to generate defects to create channels for the formation of the filaments. Thermal annealing may be used at any stage of the process to improve device switching characteristics. Ion implantation can be implemented before the SOI process and/or after the SOI process, and the depth of the ion implantation can be through the switching/host layer(s) 1020 and/or partially through the switching/host layer(s) 1020, and/or into the bottom cladding layers 524. Thin SiO₂ layer(s) can be used on one or on both the top and the bottom of the switching/host layer 1020, as illustrated in FIG. 10, to facilitate wafer bonding of the Si to the switching/host layer(s) 1020. Epitaxial lateral overgrowth of Si over the dielectric switching layers can also be used to bury the switching layer/host between Si cladding layers.

In certain cases, it is preferred that the metal electrodes are not in direct contact with the switching layers where migration of the metal ions into the switching layers can result in degradation of reliability and lifetime. In certain cases the switching ions are ion implanted, and/or deposited by other means such as co-deposition of the switching ions during deposition of the switching layer, doping of the switching layer with switching ions and vacancies, for example. Density of the switching ions can range from 1×10¹⁶/cm³ to 1×10²²/cm³ or greater. Ions can include the metals, non-metals, the transition metals, post-transition metals, alkali earth metals, alkali metals, Lanthanide, and other non-metals such as fluorides, carbides, sulphides, in addition to oxygen, nitrogen, fluorine, chlorine, hydrogen, argon, xenon, and helium.

Other methods of fabricating a buried electrically insulating layer in silicon and/or silicon germanium alloy in addition to wafer bonding, epitaxial lateral overgrowth, selective area growth, can include nanostructures where thermal oxidation can fill a void and the thermally grown silicon dioxide can fill the void completely since SiO₂ occupies a larger volume than Si.

FIGS. 11A-11C and 12A-12B are cross section drawings illustrating aspects of manufacturing memory cells, according to some embodiments. In FIG. 11A, the nanostructures 1102 can be etched into Si 1110 and/or using Si epitaxial lateral overgrowth (ELOG) over a sacrificial dielectric layer 10 nm thick for example and then etching away the sacrificial dielectric layer leaving a 10 nm gap in height. The silicon 1110 can be n doped, p doped or undoped intrinsic. In FIG. 11B thermal oxide can be grown such that the voids between nanostructures 1102 are filled with silicon dioxide 1104 partially and/or completely as shown. FIG. 11C shows separation of the memory cells by a trench etch 1140. Planarization and/or polishing such as CMP can be used in the process to form the structure illustrated in FIG. 11C.

FIG. 12A illustrates ion implantation 1252 into the silicon dioxide with species that can cause damage 1250 for resistive switching and/or conductive bridging. The implant step of FIG. 12A can also be performed prior to isolation trenching as in FIG. 11C and can be a blanket ion implantation and/or localized ion implantation where an ion implantation mask is used such that areas to be implanted will not have the mask material. Following ion implant, optional thermal anneal can be performed before or after the deposition of top and bottom electrodes TE and BE shown in FIG. 12B. Not all steps for a compete RRAM device are shown for clarity. Only certain steps are shown to illustrate an example of a way of achieving a buried oxide with Si cladding. The thermal oxide thickness can range from 0.5 nm to 100 nm. Thermal oxide consumes approximately 45.5% silicon, so that a 10 nm silicon dioxide consumes about 4.55 nm silicon. Thus, in FIG. 11B a nanostructure with a gap of 10 nm can be thermally oxidized to fill the gap completely and/or partially with oxide growing from top and bottom simultaneously and consume approximately 2.27 nm of Si from each Si surface. The gap can range from 1 nm to 100 nm for example.

Ion implant of species ions can range in dose from 1×10⁶ ions/cm² to 5×10¹⁵ ions/cm² or higher, and in some cases to 5×10¹⁷ ions/cm² or higher. Acceleration energies can range from less than 1 KeV to 400 KeV or more and the switching layer thickness can range from 1 nm to 100 nm or more. Ions can consist of the transition metals, post transition metals, H, N, O, He, Ar, Ne, Xe, Cl, S, F, to name a few. In addition, or instead, radiation can be used to generate defects and vacancies from gamma rays to x rays to UV for example.

In certain cases, where the switching layer is buried and cladded by silicon and or another semiconducting material such as Ge, SiGe, III-V material family, top and bottom (or left and right) and where the doping of ions is by methods such as atomic layer deposition for example, ion implantation and or radiation may or may not be further necessary for reproducible reliable RRAM operation.

In addition, other material such as SiGe, Ge, III-V family material such as GaAs, InP, GaN, to name a few can also be used in combination. Purity of the material can be critical for controlling the reproducibility and reliability of the resistive random access memory. In some cases, the purity of the Si should be equivalent to the purity required for CMOS electronic device fabrication and processing, and in some cases the purity of the Si should be 99.9999999% pure or better, other than dopants such as donors and acceptors that are introduced intentionally for device operation. The purity standard applies to other materials used for the RRAM device. In a RRAM device, dopants, switching ions/species, vacancies/defect density are generated through ion implantation/radiation and/or epitaxial growth of CMOS standard in quality and purity and doping concentration.

For example there are approximately 5×10²² Si atoms per cm³ and if background impurities are less than 10¹⁵/cm³, then the purity of the silicon is better than 0.9999999; in some cases the background impurities can be less than 10¹³/cm³ and the purity of the silicon can be better than 0.999999999; in some cases the background impurity is less than 10¹²/cm³ and the purity of the silicon can be better than 0.9999999999; and in some cases the background impurity is less than 2×10¹⁰/cm³ and the silicon purity can be approximately 0.999999999999 for example.

In known techniques, such control in purity and concentration in the switching layer is lacking and this causes poor reproducibility and reliability. To date, there is no known commercialization of a RRAM array on a high volume scale such as for flash memories for example.

The preferred material is Si and silicon dioxide, silicon nitride, silicon carbide, high dielectric gate materials such as hafnium oxide used in CMOS and combination thereof, and the RRAM array in 2D and/or 3D can be monolithically integrated with CMOS and/or BiCMOS electronics into a single chip. 3D refers here to stacking RRAM devices such as those illustrated in FIG. 1D or FIG. 10 for example, such that there are vertical stacks of respective RRAM elements.

In certain cases, metal ions from electrodes may migrate to the switching/host layer causing unintentional doping of metal ions into the switching layer. The density of such unintentionally migrating ions into the switching/host layer(s) should be less than 10²²/cm³ and in some cases less than 10²⁰/cm³ and in some cases less than 10¹⁸/cm³ and in some cases less than 10¹⁶/cm³ and in some cases less than 10¹⁴/cm³ and in some cases less than 10¹³/cm³. It is desirable that the unintentional ions are at least 10 times lower density than the intentional ions density, and in some cases 50 times lower and in some cases 100 times lower and in some cases 1000 times lower and in some cases more than 1000 times lower. There are cases where metal ions, O ions, vacancies, N ions, to name a few that are intentionally doped either by growth and or implantation into regions outside of the switching/host layer(s), from which they can migrate into the switching/host layer(s) to participate in the resistive switching process.

Reliability for the purposes of this patent specification means that RRAM devices have an expected lifetime of at least 108 cycles of switching when manufactured using current commercial manufacturing practices of companies such as one or more of Intel, Micron, and TSMC, and reproducibility means that RRAM devices can be manufactured using current commercial manufacturing practices of companies such as one or more of Intel, Micron, and TSMC with a yield of at least 90% in a run in which the individual elements of a RRAM device differ from each other in switching voltages when supplied to a customer or distributor by less than 10% and different RRAM devices of a manufacturing run differ from each other in switching voltages when supplied to a customer or distributor by less than 10%.

Purity standards of commercial CMOS for the purposes of this patent specification means the purity standards for relevant materials in CMOS devices currently supplied commercially on a large scale by companies such as one or more of Intel, Micron, and TSMC.

This patent application refers to certain theories in explaining the nature and operation of devices, but it should be clear that such theories are based on current understanding and do not affect the actual operation of the disclosed devices even if future developments prove the theories incorrect. This patent specification also refers to numerical ranges of parameters, and it should be understood that insubstantial departures from such ranges are still within the spirit of the disclosed advancements.

Although the foregoing has been described in some detail for purposes of clarity, it will be apparent that certain changes and modifications may be made without departing from the principles thereof. It should be noted that there are many alternative ways of implementing both the processes and apparatuses described herein. Accordingly, the present embodiments are to be considered as illustrative and not restrictive, and the body of work described herein is not to be limited to the details given herein, which may be modified within the scope and equivalents of the appended claims. 

What it claimed is:
 1. A resistive random access memory device (RRAM) comprising: a switching layer that in operation of the RRAM changes between higher resistance state (HRS) and lower resistance state (LRS) upon application across the layer of a voltage that is below a breakdown voltage; said switching layer comprising metal ions and defects introduced therein by ion implantation or radiation; and a first electrode at one side of the switching layer and a second electrode at an opposite side of the switching layer, positioned to apply said voltage in said operation of the RRAM; wherein said electrodes are configured to keep migration of metal ions from the electrodes to the switching layer from degrading reliability or reproducibility of the RRAM.
 2. The resistive random access memory device of claim 1, in which the density of metal ions in the switching layer due to intentionally introduced ions and ions that have migrated during an expected lifetime of the RRAM is at a selected level in the range 10¹³ to mid-10²² ions/cm.sup.3.
 3. The resistive random access memory device of claim 2, in which the selected level is no greater than mid-10²² ions/cm.sup.3.
 4. The resistive random access memory device of claim 2 in which the electrodes are made of material from which metal migration to the switching layer is sufficiently low to maintain the density of metal ions in the switching layer below said selected level.
 5. The resistive random access memory device of claim 2, in which the switching layer initially is substantially stoichiometric except for said metal ions that are at a density below the selected level.
 6. The resistive random access memory device of claim 1, further including a respective cladding layer between each of the electrodes and the switching layer that spaces the electrodes from the switching layer and thus inhibits migration of ions from the electrodes to the switching layer.
 7. The resistive random access memory device of claim 6, in which the electrodes are spaced from edges toward centers of sides of the cladding layers, thus reducing undesirable electric field effects at said edges.
 8. The resistive random access memory device of claim 6, in which the switching layer is SiO₂ and the cladding layers are Si.
 9. The resistive random access memory device of claim 1, in which the switching layer comprises a plurality of layers of respective different materials.
 10. The resistive random access memory device of claim 1, in which the switching layer is between 1 and 100 nanometers (nm) thick and each cladding layer is between 2 and 2000 nm thick.
 11. The resistive random access memory device of claim 1, including a cladding layer between at least one of the electrodes, which cladding layer is doped to create a PN junction.
 12. The resistive random access memory device of claim 1, in which the switching layer is divided into an array of individually addressable RRAIVI elements.
 13. The resistive random access memory device of claim 1, further including a cladding layer between at least one of the electrodes and the switching layer, wherein said metal ions and/or defects are introduced in said switching layer by ion implantation or radiation through said cladding layer.
 14. The resistive random access memory device of claim 1, in which at least one of the electrodes comprise a metal or a metal alloy and has a side facing and in contact with the switching layer and a region bound by said side, wherein free metal ions are absent from said region except for possible trace amounts.
 15. A resistive random access memory device (RRAM) comprising: a switching layer that switches between higher resistance state (HRS) and lower resistance state (LRS) upon application across the layer of a voltage that is below a breakdown voltage; a first cladding layer at one side of the switching layer and a second cladding layer at an opposite side of the switching layer; and a first electrode on the first cladding layer and a second electrode on the second cladding layer; wherein said electrodes are out of physical contact with the switching layer.
 16. The resistive random access memory of claim 15, in which the electrodes comprise metal or metal alloy.
 17. The resistive random access memory device of claim 15, in which the switching layer is substantially stoichiometric except for metal ions introduced in initially stoichiometric material in manufacturing the RRAM to cause said switching between the HRS and LRS states and any ions migrating to the switching layer from the electrodes or the cladding layers during operation of the RRAM.
 18. The resistive random access memory device of claim 17, in which the density of said ions in the switching layer is below mid-10²² metal ions/cm³.
 19. The resistive random access memory device of claim 15, in which the switching layer is SiO₂ and the cladding layers are Si.
 20. The resistive random access memory device of claim 15, in which the switching layer comprises a stack of layers of different materials.
 21. The resistive random access memory device of claim 20, in which at least two layers in the stack differ in thickness from each other.
 22. The resistive random access memory device of claim 20, in which at least two layers in the stack comprise oxides.
 23. The resistive random access memory device of claim 15, in which the switching layer comprises switching ions that directly participate in said switching and non-switching ions that do not directly participate in the switching but cause damage or vacancies in the switching layer.
 24. The resistive random access memory device of claim 23, in which the cladding layers contain no more than 10¹³ impurity atoms or ions per cm³ prior to introduction of switching ions into the switching layer and excluding p and n doping ions in the cladding layers.
 25. The resistive random access memory device of claim 15, in which the switching layer is between 1 and 100 nm thick and each cladding layer is between 2 and 2000 nm thick.
 26. A resistive random access memory device (RRAM) comprising: a switching layer that switches between higher resistance state (HRS) and lower resistance state (LRS) upon application across the layer of a voltage below a breakdown voltage; wherein said switching layer is substantially stoichiometric except for ions introduced in manufacturing the RRAM that are sufficient to cause the layer to switch between the HRS and LRS states and ions introduced due to operation of the RRAM; a first cladding layer at one side of the switching layer and a second cladding layer at an opposite side of the switching layer; and a first electrode on the first cladding layer and a second electrode on the second cladding layer.
 27. The resistive random access memory device of claim 26, in which the total density of ions in the switching layer causing switching between the HRS and LRS states, including ions introduced in manufacturing and ions migrated from the electrodes or the cladding layers in operation of the RRAM, does not exceed 10²² ions/cm³.
 28. The resistive random access memory device of claim 27, in which the switching layer comprises two or more layers of different materials.
 29. The resistive random access memory device of claim 27, in which the switching layer comprises two or more layers that have different densities of metal ions.
 30. The resistive random access memory device of claim 26, in which at least one of the electrodes comprises a metal or a metal alloy and is spaced from the switching layer by a distance inhibiting migration of metal ions to the metal or metal alloy to the switching layer.
 31. The resistive random access memory device of claim 26, in which said ions and/or defects introduced in manufacturing the RRAM are introduced by ion implantation or radiation through one of the cladding layers and into said switching layer.
 32. The resistive random access memory device of claim 26, in which said ions and/or defects introduced in manufacturing the RRAM are introduced by ion implantation or radiation through one of the cladding layers and into and past said switching layer.
 33. The resistive random access memory device of claim 26, in which the switching layer thickness is in the range of 1 to 100 nm.
 34. The resistive random access memory device of claim 26, in which each of the cladding layers is between 2 and 2000 nm thick.
 35. The resistive random access memory device of claim 26, in which the switching layer comprises one or more oxides, nitrides, or carbides selected from the group of silicon dioxide, aluminum oxide, titanium oxide, tantalum oxide, hafnium oxide, vanadium oxide, tungsten oxide, molybdenum oxide, niobium oxide, chromium oxide, nickel oxide, transition metal oxides, silicon nitride, niobium nitride, tungsten nitride, gallium nitride, aluminum nitride, all metal nitrides, silicon carbide, hafnium carbide, magnesium carbide transition metal carbide, and metal carbides.
 36. The resistive random access memory device of claim 26, in which at least one of the cladding layers comprises one or more material selected from the group consisting of Si, Ge, SiGe, GaAs, GaN, AlN, InP, InN, AlGaN, and the III-V material family.
 37. The resistive random access memory device of claim 26, in which the electrodes include non-trace amounts of one or more of Al, aluminum silicide, platinum silicide, nickel silicide, titanium silicide, magnesium silicide, tungsten silicide, molybdenum silicide, platinum, titanium, tungsten, tantalum, copper, and other metals. 